The invention relates to a spray nozzle for spraying a continuous casting product with a cooling liquid according to the preamble of claim 1.
As is known, in a continuous casting process, in particular for the continuous casting of steel, cooling of a metal melt in a continuous casting mould results in a continuous casting product which is continuously drawn out of the mould in the form of a strand whose surface is constituted by a solidified crust and which still has a liquid core of metal melt. After leaving the mould, the strand is conveyed through a secondary cooling zone in which it is sprayed with a coolant, generally water, in order to continue removing heat from it until it has completely solidified and bring it to the temperature desired for subsequent processing.
As secondary cooling directly causes the strand to solidify or influences its solidification, the secondary cooling process and the devices required to carry it out have a decisive effect on the quality of the end products. The components used to disperse the coolant, in particular the spray nozzles, are of particular importance.
The various parameters which characterise the secondary cooling process affect the solidification of the strand in different ways andxe2x80x94depending on the practicexe2x80x94must be optimised according to different criteria.
Particularly important factors are the secondary cooling intensity, which determines the speed of the strand shell growth and which is set to be more or less xe2x80x9charshxe2x80x9d or xe2x80x9cgengentlexe2x80x9d, depending on the practice, and the spatial distribution of the coolant application density, which should be as homogeneous as possible in order to ensure that the strand shell growth is as homogeneous as possible.
The spray nozzles used in a secondary cooling section to atomise a coolant are usually optimised with regard to the required standards of secondary cooling intensity and homogeneity of the coolant application. The kinetic energy of the cooling liquid droplets applied by spraying and, in particular, the coolant application density are in this respect determining factors for the secondary cooling intensity. The homogeneity of the coolant application density is not just determined by the homogeneity of the droplet dispersion in the spray jet produced by an individual spray nozzle. The angular distribution of the droplet paths is also relevant to the homogeneity of the coolant application density. Namely, the angular distribution determines the shape and the size of the area on a strand which can be sprayed with a spray jet. However a large number of spray nozzles are required in a secondary cooling zone in order to cover with coolant the entire area of a strand which is to be cooled. The spray jets of the individual nozzles are therefore superimposed accordingly. The angular distribution of the droplet paths of an individual spray jet is consequently a decisive factor for the homogeneity of the coolant application density when superimposing a large number of spray jets.
The known full cone nozzles deliver spray jets with a conical angular distribution of the droplet paths. Because of their conical shape, the spray jets of a plurality of full cone nozzles are unable to perfectly cover large areas for spraying; the superimposition of a plurality of spray jets results in a highly inhomogeneous coolant application density. A spray nozzle having all the features of the preamble of claim 1 is known from U.S. Pat. No. 3 072 346. This spray nozzle has a nozzle body with a mixing chamber which is rotationally symmetrical about the longitudinal axis of the nozzle body, into which chamber a liquid, forming a first and a second liquid stream, can flow through two inlet openings and which is provided with an outlet opening, disposed downstream, for a spray jet. Apart from the formation of the outlet opening, this nozzle has essential features of a known type of full cone nozzle: The two inlet openings are integrated into a guide structure for the liquid streams entering the mixing chamber such that a velocity component is imparted to the liquid streams tangentially to the mixing chamber wall as they enter the mixing chamber in addition to a velocity component in the direction of the outlet opening. This tangential velocity component causes the two liquid streams to combine after entering the mixing chamber to form one liquid stream which is directed at the outlet opening and which exhibits a vortex about the longitudinal axis of the nozzle body. Although the spray nozzle which is described in U.S. Pat. No. 3 072 346 has a round outlet openingxe2x80x94like a conventional full cone nozzlexe2x80x94, this outlet opening is widened like a funnel on the outlet side such that the emerging spray jet is distorted in the direction of the diagonals of a square. Because the outlet opening is formed in this way, the nozzle delivers a spray jet with an approximately square droplet dispersion related to a plane perpendicular to the longitudinal axis of the nozzle body.
One disadvantage of this spray nozzle is that, because of the vortex which is imposed, the form of the droplet dispersion of the spray jet is distorted to an increasing degree as the infeed pressure of the liquid increases. It is therefore impossible to comply with the standards required in terms of homogeneity of the coolant application density in a secondary cooling section with a nozzle of this kind.
A further disadvantage of this nozzle lies in the fact that its spray jet only has an approximately square droplet dispersion in one spray plane, which may not be very far away from the outlet opening, typically not more than 20 cm. Because of the short operating distance, a large number of spray nozzles of this kind are required to spray large areas with a sufficient degree of homogeneity.
A flat-jet nozzle is described in U.S. Pat. No. 4 988 043. It comprises a passage for the liquid to be atomised with an outlet slot for the spray jet. The spray jet is fanned out over a wide angular range in the slot direction, whereas it hardly widens transversely to the longitudinal direction of the slot as the distance from the outlet slot increases. The quasi one-dimensional fan-out results in a flat spray jet. On account of the small extent of the spray jet transversely to the outlet slot, the process of spraying relatively large rectangular areas entails complications, whether because a large number of these flat-spray nozzles must be used or because a single flat-spray nozzle must be moved in order to cover a relatively large area with its spray jet.
Taking the inadequacies of the known spray nozzles as a starting point, the object of the invention is to provide a spray nozzle which is suitable for use in a secondary cooling section of a continuous casting plant and for this purpose enables the largest possible area to be sprayed as homogeneously as possible with liquid droplets with the greatest possible kinetic energy from the greatest possible distance.
This object is achieved by a spray nozzle having the features of claim 1.
The spray nozzle according to the invention comprises a mixing chamber into which a liquid, forming a first and a second liquid stream, can flow through two inlet openings and which comprises an outlet opening, disposed downstream, for a spray jet, wherein at least one mixing chamber wall is formed as a guide surface for the liquid streams and is shaped at the outlet opening such that the liquid streams meet at an angle at or directly before the outlet opening and then form the spray jet. Because the two liquid streams are directed at the outlet opening and collide at the outlet opening, relatively large liquid droplets are produced whichxe2x80x94related to the infeed pressure at the inlet openingsxe2x80x94can leave the outlet opening with a relatively high level of kinetic energy. Energy losses due to vortex formation in the mixing chamber are largely prevented. The high level of kinetic energy allows an area to be sprayed from a considerable working distance. The atomisation of the two liquid streams permits a large spread of the directions of propagation of the droplets and therefore a wide fan-out of the spray jet emerging from the outlet opening. In this respect the droplets which are dispersed transversely to the direction of propagation of the liquid streams in particular play an important part in the fan-out of the spray jet. As the propagation of the liquid streams in the mixing chamber is substantially determined by the geometry of the mixing chamber, the infeed pressure can be varied over a relatively broad range without any substantial change in the fan-out of the spray jet.
In this connection the cross section of an inlet opening is basically understood to mean a section transverse to the respective liquid stream in the inlet opening, and the cross section of the outlet opening a section transverse to the spray jet.
The properties of a spray jet produced with the spray nozzle according to the invention depend substantially on the angle of impact at which the liquid streams meet at or directly before the outlet opening. An angle of impact in a range between 60xc2x0 and 130xc2x0, preferably between 80xc2x0 and 100xc2x0, is advantageous. This creates the conditions for producing liquid droplets which leave the outlet opening with a particularly high level of kinetic energy and form a spray jet which is distinguished by the fact that the droplets disperse in a particularly uniform manner over a particularly large solid angle about a mean direction of propagation.
The spray nozzle according to the invention has a slot as the outlet opening. If its cross-sectional area transverse to the direction of propagation of the spray jet is appropriately shaped, an outlet slot offers the possibility of spraying a rectangular area, for example. The long sides of the rectangular area for spraying are in this case substantially parallel to the direction of the longitudinal extent of the slot. The angular range over which the spray jet fans out in the direction of the longitudinal extent of the outlet slot increases with the length of the slot. This effect is due to the fact that the angular range in which droplets can leave the interaction zone of the two liquid streams at the outlet opening through the outlet slot increases in the direction of the longitudinal extent of the slot with the length f the outlet slot.
In one embodiment of the spray nozzle according to the invention the mixing chamber has a taper at the outlet opening with an opening angle at the outlet opening of between 60xc2x0 and 130xc2x0, preferably between 80xc2x0 and 100xc2x0. The taper forms the part of the guide surface for the liquid streams which determines the angle of impact. The taper brings together the two liquid streams at the outlet opening at an angle of impact which corresponds to the opening angle of the taper. The droplets produced at the outlet opening when the two liquid streams interact have a particularly large velocity component in the direction of the bisector of the opening angle of the taper. This direction corresponds to the mean direction of propagation of the droplets which can leave the outlet opening. Depending on its shape, the outlet opening also provides an exit for droplets whose paths are scattered over a solid angle about the mean direction of propagation. The taper may be conical, for example.
A number of further developments of the spray nozzle according to the invention have further features which, either alone and/or combined with one another, afford the condition for a homogeneous droplet dispersion over an area for spraying. In order to achieve a homogeneous droplet dispersion, it is advantageous for the outlet opening and the mixing chamber to have a common plane of symmetry. On this assumption, the two liquid streams are symmetrical with respect to the plane of symmetry. Droplets whose paths extend symmetrically to the plane of symmetry can therefore be produced. A spray nozzle whose outlet opening is formed as a slot will produce a particularly homogeneous droplet dispersion if the inlet openings in each case have a cross-sectional area of elongate shape and the directions of their longitudinal extent are in each case substantially parallel to the direction of the longitudinal extent of the outlet slot. In this case the two liquid streams are in a sense xe2x80x9cpreshapedxe2x80x9d and adapted to the outlet slot at the inlet openings so that even at the inlet openings the lines of equal flow velocityxe2x80x94related to a plane transverse to the respective liquid streamxe2x80x94have the same or approximately the same shape as the cross-sectional area of the outlet opening (transversely to the central direction of propagation of the liquid droplets).
Another embodiment of the spray nozzle according to the invention has an outlet slot and is formed such that the mixing chamber and the outlet slot have a common plane of symmetry, wherein the longitudinal direction of the outlet slot lies in the plane of symmetry and the inlet openings are disposed on different sides of the plane of symmetry. In this case the spray jet is fanned out particularly widely in the plane of symmetry, i.e. in the longitudinal direction of the outlet slot. The droplet dispersion additionally becomes particularly homogeneous ifxe2x80x94as in the previously discussed embodimentxe2x80x94the inlet openings have a cross-sectional area of elongate shape and the directions of their longitudinal extent are substantially parallel to the plane of symmetry. A particularly uniform droplet dispersion is achieved if the ratio of the sum of the two cross-sectional areas of the inlet openings to the cross-sectional area of the outlet opening is between 1.5 and 2, preferably between 1.6 and 1.8.
Another embodiment of the spray nozzle is distinguished by the fact that the mixing chamber has a taper of the above-mentioned type at the outlet opening and a cylindrical segment between the taper and the inlet openings. The cylindrical segment acts as a side wall which bounds the liquid streams. The length of the cylindrical element influences the way in which the two liquid streams intermix at the outlet opening and the efficiency with which the liquid streams are converted into droplets which leave the outlet opening unimpeded. The length of the cylindrical segment may be optimised accordingly. Additionally, it is advantageous if the inlet openings open out at the side wall of the mixing chamber. Then the energy losses due to unwanted vortex formation in the mixing chamber are particularly low and the production of the spray jet is particularly efficient.
A spray nozzle with a mixing chamber of a particularly simple construction is obtained if the inlet openings are formed between a cross bar, which connects opposite parts of the lateral boundary of the liquid streams, and the lateral boundary. If the side wall is rotationally symmetrical about an axis and the cross bar is cuboid, the cross sections of the inlet openings will be shaped like circular segments. According to the invention such inlet openings may be combined with an outlet slot whose longitudinal direction is substantially parallel to the chords of the circular segments.
The droplet dispersion in the spray jet may be influenced by defined widenings of the cross section of the outlet opening in the direction of propagation of the spray jet. One embodiment of the spray nozzle according to the invention has an outlet slot whose cross-sectional area is widened at the narrow ends in the direction of propagation of the spray jet. A particularly large fan-out of the spray jet in the longitudinal direction of the outlet slot is achieved by this means.
In another embodiment of the spray nozzle the cross section of the outlet slot is widened in the centre of the long sides of the outlet slot in the direction of propagation of the spray jet. This measure enables the proportion of droplets propagating in the direction of the mean direction of propagation to be increased.
In another embodiment of the spray nozzle according to the invention the outlet opening and the mixing chamber have a common plane of symmetry, and guide walls are provided to bound the spray jet emerging from the outlet opening.
In another embodiment of the spray nozzle according to the invention the spray nozzles are asymmetrical in that the inlet openings have different cross-sectional areas and/or the guide walls are disposed on opposite sides of the outlet opening at a different distance from the latter. These two constructional measures result on the inlet and/or outlet side in an asymmetry of the spray nozzle which has an effect on the droplet dispersion in the spray jetxe2x80x94even if the mixing chamber is otherwise symmetrical. If this asymmetry is suitably prominent in quantitative terms, it is possible, in comparison with a symmetrical nozzle, to move the centroid of the droplet dispersion by a predetermined distance, influence the homogeneity of the droplet dispersion and vary the shape of the area for spraying. It is possible, inter alia, to form areas for spraying with more or less curved circumferential linesxe2x80x94instead of a rectangular area for spraying. A spray nozzle whose mixing chamber comprises a plane of symmetry will produce a particularly homogeneous droplet dispersion on a rectangular area for spraying with a centroid which is offset with respect to the plane of symmetry if the nozzle is formed asymmetrically on the inlet and the outlet side such that the inlet opening with the smaller cross-sectional area is disposed on the same side of the plane of symmetry as the guide wall which is disposed at the greater distance from the plane of symmetry. To achieve optimum results, the distances of the guide walls from the plane of symmetry may be adapted to the asymmetry of the nozzle on the inlet side, which is characterised, for example, by the difference in size of the cross-sectional areas of the inlet openings.
A spray nozzle according to the invention which is provided with a suitable outlet slot enables, for example, a rectangular area of a width of 10 cm and a length of 50 cm to be uniformly sprayed from a distance of approximately 45 cm. Spray nozzles of this kind may advantageously be used in a secondary cooling section of a continuous casting plant to cool strands of billet or bloom format, in which case one of the spray nozzles would replace 4-6 conventional full cone nozzles and in addition make a more uniform coolant application possible. The nozzle according to the invention may be constructed with an outlet slot of a length exceeding 10 mm and a width exceeding 5 mm. Openings of this size entail little risk of the outlet slot of the spray nozzle according to the invention becoming clogged due to soiling during operation, which is quite the opposite of conventional spray nozzles. The same applies to the inlet openings, which may be approximately of the same size as the outlet openings.
The asymmetrical embodiments of the spray nozzle according to the invention are used in various ways in a continuous casting plant. In a curved mould continuous casting plant, for example, portions of a curved strand with a rectangular cross section are cooled on the different sides in the region of the secondary cooling zone by superimposing areas for spraying in the form of rectangles and segments of circular rings. Such areas can be generated with the spray nozzle according to the invention by appropriately dimensioning the components thereof. Moreover, it is usual, when casting parts in succession, to vary the cross section of the strands to be produced. This results in the problem that, after changing the cross section in a longitudinal portion of a strand path, it is not just the size of an area for spraying which has to be adapted to the changed strand geometry, but also frequently the centroid of this area. When using conventional spray nozzles, these must all be replaced by other nozzles with a different area for spraying when the cross section is changed, in which case the position of the spray nozzles must also be appropriately adapted. The same object can be achieved by means of the spray nozzle according to the invention by positioning the spray nozzles at a predetermined point and optionally using spray nozzles with differing asymmetry which take account of the change in the centroids of the areas for spraying. This procedure eliminates the complex step of re-adjusting the spray nozzle each time the cross section is changed.